targeted mrsi via fully-refocused spatiotemporal imaging ...€¦ · pc 180° pulses is combined...
TRANSCRIPT
Magnetic resonance spectroscopic imaging (MRSI)
plays numerous roles in contemporary research.
Recently, SPatiotemporal ENcoding (SPEN) [1]
provides an alternative to EPI with similar
acquisition durations, resolution and sensitivity
parameters while providing enhanced immunity to
artifacts such as B0 inhomogeneities and
susceptibility effects. Particularly robust are fully-
refocused SPEN experiments [2], providing a
voxel-by-voxel refocusing of all frequency shifts in
the sample along the PE domain. While this
removes all shift information, we have recently
shown how polychromatic (PC) pulses targeting a
priori resonances, can be used to phase modulate
and eventually separate peaks from different sites
[3]. In this study, this Fourier Encoding based on
PC 180° pulses is combined with fully-refocused
SPEN experiments, to provide a PC-SPEN MRSI
sequence that is both efficient and robust for the
rapid acquisition of spectroscopic images.
Results
Targeted MRSI via Fully-Refocused Spatiotemporal Imaging and
Polychromatic Spectral Pulses (PC-SPEN)Zhiyong Zhang1, 2; Lucio Frydman1
1Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel; 2Department of Electronic
Science, Xiamen University, Xiamen, Fujian 361005, China;
The PC-SPEN MRSI sequence is shown in Fig. 1a. The PC 180°
pulse following the adiabatic sweep 180° pulse and timed as
indicated, provides full refocusing [2] while addressing N resonances
of interest according to PCm = Nm = 1Pn
180(Ωn)eiπmn/N, where 0 ≤ n ≤ N-1,
m is a scan index among M ≥ N, and Pn180(Ωn) is a selective 180°
pulse centered at the frequency of the n-th resonance, Ωn. This
manipulation behaves as Fourier encoding, imparting a phase
modulation ei2πmn/N into n-th chemical shift component in m-th scan.
After Fourier transforming these scans’ signals, all chemical shifts –
as well as their images – are separated. Moreover, this PC pulse
restores to equilibrium all spins that were not targeted by the initial
slice-selective excitation, enabling multi-slice spectroscopic imaging.
Notice that, for the different decoded image components there will a
chemical shift miss-registration along SPEN’s low-bandwidth
dimension, which can be corrected using the corresponding
chemical shift information. All the SPEN images are processed with
a referenceless super-resolution (SR) reconstruction algorithm [4].
MethodsIntroduction
The usefulness of this PC-SPEN
MRSI approach is demonstrated
on a metabolite phantom (Fig. 2)
and in vivo water-fat separation
imaging at 7 T (Figs. 3 and 4).
(See captions for details) The
phantom is composed of three
tubes containing separate choline
(III: Cho, 50 mM), N-Acetyl-L-
Aspartic acid (II: NAA, 250 mM)
and sodium lactate (I: Lac, 125
mM) solutions, immersed in a
fourth, 15 mm diameter water
tube.
Conclusion: The PC-SPEN MRSI provides a robust technique to map multiple chemical shift images with high time efficiency.
Fig. 4 In vivo fat/water separation comparisons
of the PC-SPEN and PC-SE EPI multi-slicing
methodologies, as applied to abdominal mouse
investigations at 7T. (a) Water/fat separation
from 10 slices arising from a PC-SPEN
sequence with M = 3. (b) Idem but from a PC-
SE-EPI sequence. Common parameters of
these images were as in Fig. 3; the total scan
duration for the PC-SPEN scan was 30 sec,
while the PC-SE-EPI scan required 60 sec (the
additional time stemming from EPI’s reliance on
reference scans for joint co-processing of
even/odd phase-encoded lines). It shows more
robustness of PC-SPEN to distortions
Fig. 3 In vivo fat/water separation using the PC-SPEN
methodology, applied to abdominal mouse imaging at 7T. (a, b)
Scout images for the 5-slice selection, the water-derived slices
are marked in red lines while the fat-derived slices are marked in
green lines. (c, d) The 3rd multiscan spin echo references
involving fat suppression (c) and water suppression (d). (e) The
spectra acquired using the sequence with slice selection pulse
followed by PC pulses in three scans and well-separated
spectra after applying a Fourier transform (PC decoding) on the
encoded spectra. (f) The water/fat separation using PC-SPEN
sequence, applied to the 5 slices. Common parameters of these
images: FOV =40 40 mm2; slice thickness = 2 mm; bandwidth
of chirped pulse =10.9 kHz; PC pulse duration = 5 ms . The total
scan time for PC-SPEN is 15 sec using 3 scans with PC phase
modulation. The edges’ decay along SPEN dimension in PC-
SPEN images is due to the chirp encoding profile has rising and
falling edges.
Acknowledgments: This research was funded by Israel Science Foundation grant 795/13, by ERC PoC Grant #633888, by the Kimmel Institute for Magnetic Resonance (Weizmann Institute),
and by the generosity of the Perlman Family Foundation. ZZ is thankful for financial support from the China Scholarship Council (201306310056).
References: [1]. A. Tal, L. Frydman, Prog. Nucl. Magn. Reson. Spectrosc., 2010, 57:241-292. [2]. R. Schmidt, and L. Frydman, Magn. Reson. Med., 2014, 71:711-722. [3]. Z.Y. Zhang, P. Smith
and L. Frydman, J. Chem. Phys. 2014, 141:.194201. [4] A. Seginer, R. Schmidt, A. Leftin, et al., Magn. Reson. Med., 2014, 72:1687-1695.
The 23rd ISMRM Conference, Toronto, Ontario, Canada, May 30 ~ June 5, 2015
Fig. 1 (a) PC-SPEN sequence, using a phase-modulated polychromatic
(PC) pulse in SPEN sequence, in order to enable the separation of the
spectral peaks. (b) Analogous PC-SE EPI sequence, replacing SE-EPI’s
refocusing 180˚ pulse with a PC counterpart. (c) Purely spectroscopic PC-
FID sequence using a PC pulse following a slice selection.
FOV = 20×20 mm2; matrix size = 64×64; slice
thickness = 4 mm; Ta = 24.6 ms; bandwidth of the
180˚ adiabatic encoding pulse = 4.88 kHz; PC pulse
duration = 20 ms; PC phase increments M = 6;
repetition time TR = 2 sec. Total scan time was 25
min, including the averaging of 128 PC-encoded
experiments for SNR improvement.
Fig. 2 Implementing PC-SPEN MRSI in the phantom (a)
six SPEN images modulated by PC pulses imparting the
desired FT encoding on three metabolic components. (b)
Results arising from an inverse FT of the 6 images shown
in (a): panels b-1~3 show the first three decoded images,
while b-4 is a sum image. (c) Idem as (b), but after the (b)
images were corrected by shift-derived miss-placements.
Marked by squares in (c-4) are regions of interests for
calculating the residual cross-talk level. Scan parameters: